Phospholipase A2 (PLA2) is a key enzyme in the production of diverse mediators of inflammatory conditions, and is implicated in the pathophysiology of many diseases including arthritis, cancer, inflammatory bowel diseases, sepsis, septic shock, pancreatitis, stroke and post-surgical peritoneal adhesions. Correlation to PLA2 activity has furthermore been observed with several forms of acute and chronic brain injury, including cerebral trauma, ischemic damage, induced seizures in the brain and epilepsy, schizophrenia, and in particular, Alzheimer's disease (AD) [25]. The vast PLA2 enzyme family includes three cellular isoforms (cPLA2s), involved in signal transduction, and ten secretory isoforms (sPLA2s). The cPLA2s and sPLA2 play key roles in arachidonic acid (AA) release during acute inflammation [1]. Group IIA secretory phospholipase (sPLA2s-IIA) is known to be proinflammatory in vivo [2], and is associated with the onset of rheumatoid arthritis (RA) [3] Levels of sPLA2 in synovial fluid also correlate with severity of disease in RA patients [4]. Hence, inhibition of sPLA2 may logically block the formation of a wide variety of secondary inflammatory mediators. Moreover, sPLA2 is known as an important amplifier of cytokine-mediated prostaglandin (PG) production, and as such it plays a central role in tumour development and progression [5, 6]. Tumors from human and experimental animals produce large quantities of prostaglandins [7], which can modulate the interaction of tumor cells with various host components in cancer metastasis. Supplying PLA2 inhibitors to the cancerous tissues will therefore inhibit sPLA2 activity and reduce cyclooxygenase-2 (COX-2) expression, which in turn will suppress the catalytic conversion of arachidonic acid (AA) to PGE. This would interfere with metastasis and growth of neoplastic cells. The central role of sPLA2 in inflammatory arthritis and cancer thus makes the enzyme a potential target for drug development. It has long been recognized that inhibition of phospholipase A2 (PLA2)-catalyzed arachidonic acid (AA) release from cell-membrane glycerophospholipids could potentially block the synthesis of all eicosanoids, but this is yet to be realized.
In our previous patent filing [8] we have shown that sPLA2-IIA is inhibited by various peptide sequences derived by us from the native PLA2-inhibitory protein termed “Phospholipase Inhibitor from Python (PIP)” [9]. We have recently found that a new peptide sequence resulting from further screening of the PIP to generate inhibitors with improved potency is effective at lower micromolar concentrations in inhibiting sPLA2-IIA enzyme activity [10], and sPLA2-mediated amplification of cytokine-induced prostaglandin synthesis in mouse macrophage J774 cells in culture [11]. This new inhibitory peptide is specific and selective for sPLA2 [12]. Our in vivo experiment with transgenic mice overexpressing human TNFα further confirms the prophylactic effectiveness of this novel peptide against inflammatory arthritis in a chronic autoimmune inflammatory process [11, 13]. Despite numerous studies and accumulation of data on PLA2 inhibitors, there are no effective inhibitors presently available for clinical use either as anti-inflammatory or as anticancer agents. To date, only a few PLA2 inhibitors have progressed into clinical trials as potential drug candidates for inflammatory diseases, and unlike this short peptide they are all but organic compounds. Moreover, no sPLA2 inhibitory peptides have so far been available as drug candidates for rheumatoid arthritis (RA) or cancer treatment.
Besides the involvement of sPLA2 in RA, matrix metalloproteinases (MMPs), the zinc- and calcium-dependent family of proteins produced by synovial fibroblasts, chondrocytes, and infiltrating leukocytes [14], have been implicated in the collagen breakdown that contributes to joint destruction in RA. Several different MMPs have been shown to be present in the joints of RA patients [15]. An imbalance between the active enzymes and their natural endogenous inhibitors known as ‘Tissue Inhibitors of Metallo-Proteinases (TIMPs)’ leads to the accelerated destruction of connective tissue, which is associated with the pathology of diseases such as arthritis, cancer, multiple sclerosis and cardiovascular diseases [16]. Collagenase (MMP-1) and gelatinases (MMP-2 and MMP-9) are implicated in cancer and their inhibition has therefore been suggested as a therapeutic option [17, 18]. Accordingly, the inhibition of MMP synthesis and/or activity represents novel potential therapeutic strategies for the treatment of cancer. The potential for using specific enzyme inhibitors as therapeutic agents has therefore led to intensive research focused on the design, synthesis and molecular deciphering of low-molecular-mass inhibitors of this family of proteins. Supported by X-ray and NMR data on MMP complexes, and by exploiting sequence and structural differences in the principal specificity pocket of the enzymes, subtype-selective MMP inhibitors have been designed [19]. However, despite promising results seen in animal models, clinical trials with MMP inhibitors in cancer patients have been disappointing so far [18, 20]. Hence, the use of dual inhibitors against sPLA2 and MMPs, and in early stages of disease onset, could be a novel approach geared towards achieving improved clinical advantage over the use of MMP inhibitors alone in the treatment of cancer.